Cell Adhesion, the Backbone of the Synapse: ��Vertebrate�� and ��Invertebrate�� Perspectives

Synapses are asymmetric intercellular junctions that mediate neuronal communication. The number, type, and connectivity patterns of synapses determine the formation, maintenance, and function of neural circuitries. The complexity and specificity of synaptogenesis relies upon modulation of adhesive properties, which regulate contact initiation, synapse formation, maturation, and functional plasticity. Disruption of adhesion may result in structural and functional imbalance that may lead to neurodevelopmental diseases, such as autism, or neurodegeneration, such as Alzheimer''s disease. Therefore, understanding the roles of different adhesion protein families in synapse formation is crucial for unraveling the biology of neuronal circuit formation, as well as the pathogenesis of some brain disorders. The present review summarizes some of the knowledge that has been acquired in vertebrate and invertebrate genetic model organisms.Synapses are asymmetric, intercellular junctions that are the basic structural units of neuronal transmission. The correct development of synaptic specializations and the establishment of appropriate connectivity patterns are crucial for the assembly of functional neuronal circuits. Improper synapse formation and function may cause neurodevelopmental disorders, such as mental retardation (MsR) and autism spectrum disorders (ASD) (McAllister 2007; Sudhof 2008), and likely play a role in neurodegenerative disorders, such as Alzheimer''s disease (AD) (Haass and Selkoe 2007).At chemical synapses (reviewed in Sudhof 2004; Zhai and Bellen 2004; Waites et al. 2005; McAllister 2007; Jin and Garner 2008), the presynaptic compartment contains synaptic vesicles (SV), organized in functionally distinct subcellular pools. A subset of SVs docks to the presynaptic membrane around protein-dense release sites, named active zones (AZ). Upon the arrival of an action potential at the terminal, the docked and “primed” SVs fuse with the plasma membrane and release neurotransmitter molecules into the synaptic cleft. Depending on the type of synapse (i.e., excitatory vs. inhibitory synapses), neurotransmitters ultimately activate an appropriate set of postsynaptic receptors that are accurately apposed to the AZ.Synapse formation occurs in several steps (Fig. 1) (reviewed in Eaton and Davis 2003; Goda and Davis 2003; Waites et al. 2005; Garner et al. 2006; Gerrow and El-Husseini 2006; McAllister 2007). Spatiotemporal signals guide axons through heterogeneous cellular environments to contact appropriate postsynaptic targets. At their destination, axonal growth cones initiate synaptogenesis through adhesive interactions with target cells. In the mammalian central nervous system (CNS), immature postsynaptic dendritic spines initially protrude as thin, actin-rich filopodia on the surface of dendrites. Similarly, at the Drosophila neuromuscular junction (NMJ), myopodia develop from the muscles (Ritzenthaler et al. 2000). The stabilization of intercellular contacts and their elaboration into mature, functional synapses involves cytoskeletal arrangements and recruitment of pre- and postsynaptic components to contact sites in spines and boutons. Conversely, retraction of contacts results in synaptic elimination. Both stabilization and retraction sculpt a functional neuronal circuitry.Open in a separate windowFigure 1.(A–C) Different stages of synapse formation. (A) Target selection, (B) Synapse assembly, (C) Synapse maturation and stabilization. (D–F) The role of cell adhesion molecules in synapse formation is exemplified by the paradigm of N-cadherin and catenins in regulation of the morphology and strength of dendritic spine heads. (D) At an early stage the dendritic spines are elongated from motile structures “seeking” their synaptic partners. (E) The contacts between the presynaptic and postsynaptic compartments are stabilized by recruitment of additional cell adhesion molecules. Adhesional interactions activate downstream pathways that remodel the cytoskeleton and organize pre- and postsynaptic apparatuses. (F) Cell adhesion complexes, stabilized by increased synaptic activity, promote the expansion of the dendritic spine head and the maturation/ stabilization of the synapse. Retraction and expansion is dependent on synaptic plasticity.In addition to the plastic nature of synapse formation, the vast heterogeneity of synapses (in terms of target selection, morphology, and type of neurotransmitter released) greatly enhances the complexity of synaptogenesis (reviewed in Craig and Boudin 2001; Craig et al. 2006; Gerrow and El-Husseini 2006). The complexity and specificity of synaptogenesis relies upon the modulation of adhesion between the pre- and postsynaptic components (reviewed in Craig et al. 2006; Gerrow and El-Husseini 2006; Piechotta et al. 2006; Dalva et al. 2007; Shapiro et al. 2007; Yamada and Nelson 2007; Gottmann 2008). Cell adhesive interactions enable cell–cell recognition via extracellular domains and also mediate intracellular signaling cascades that affect synapse morphology and organize scaffolding complexes. Thus, cell adhesion molecules (CAMs) coordinate multiple synaptogenic steps.However, in vitro and in vivo studies of vertebrate CAMs are often at odds with each other. Indeed, there are no examples of mutants for synaptic CAMs that exhibit prominent defects in synapse formation. This apparent “resilience” of synapses is probably caused by functional redundancy or compensatory effects among different CAMs (Piechotta et al. 2006). Hence, studies using simpler organisms less riddled by redundancy, such as Caenorhabditis elegans and Drosophila, have aided in our understanding of the role that these molecules play in organizing synapses.In this survey, we discuss the roles of the best characterized CAM families of proteins involved in synaptogenesis. Our focus is to highlight the complex principles that govern the molecular basis of synapse formation and function from a comparative perspective. We will present results from cell culture studies as well as in vivo analyses in vertebrate systems and refer to invertebrate studies, mainly performed in Drosophila and C. elegans, when they have provided important insights into the role of particular CAM protein families. However, we do not discuss secreted factors, for which we refer the reader to numerous excellent reviews (as for example Washbourne et al. 2004; Salinas 2005; Piechotta et al. 2006; Shapiro et al. 2006; Dalva 2007; Yamada and Nelson 2007; Biederer and Stagi 2008; Salinas and Zou 2008).     

Interfering Residues Narrow the Spectrum of MLV Restriction by Human TRIM5α

TRIM5α is a restriction factor that limits infection of human cells by so-called N- but not B- or NB-tropic strains of murine leukemia virus (MLV). Here, we performed a mutation-based functional analysis of TRIM5α-mediated MLV restriction. Our results reveal that changes at tyrosine³³⁶ of human TRIM5α, within the variable region 1 of its C-terminal PRYSPRY domain, can expand its activity to B-MLV and to the NB-tropic Moloney MLV. Conversely, we demonstrate that the escape of MLV from restriction by wild-type or mutant forms of huTRIM5α can be achieved through interdependent changes at positions 82, 109, 110, and 117 of the viral capsid. Together, our results support a model in which TRIM5α-mediated retroviral restriction results from the direct binding of the antiviral PRYSPRY domain to the viral capsid, and can be prevented by interferences exerted by critical residues on either one of these two partners.     

Evolutionary Contributions to Solving the ��Matrilineal Puzzle��

Matriliny has long been debated by anthropologists positing either its primitive or its puzzling nature. More recently, evolutionary anthropologists have attempted to recast matriliny as an adaptive solution to modern social and ecological environments, tying together much of what was known to be associated with matriliny. This paper briefly reviews the major anthropological currents in studies of matriliny and discusses the contribution of evolutionary anthropology to this body of literature. It discusses the utility of an evolutionary framework in the context of the first independent test of Holden et al.'s 2003 model of matriliny as daughter-biased investment. It finds that historical daughter-biased transmission of land among the Mosuo is consistent with the model, whereas current income transmission is not. In both cases, resources had equivalent impacts on male and female reproduction, a result which predicts daughter-biased resource transmission given any nonzero level of paternity uncertainty. However, whereas land was transmitted traditionally to daughters, income today is invested in both sexes. Possible reasons for this discrepancy are discussed.     

The Clamp Loader Assembles the �� Clamp onto Either a 3�� or 5�� Primer Terminus: THE UNDERLYING BASIS FAVORING 3�� LOADING*

Clamp loaders assemble sliding clamps onto 3′ primed sites for DNA polymerases. Clamp loaders are thought to be specific for a 3′ primed site, and unable to bind a 5′ site. We demonstrate here that the Escherichia coli γ complex clamp loader can load the β clamp onto a 5′ primed site, although with at least 20-fold reduced efficiency relative to loading at a 3′ primed site. Preferential clamp loading at a 3′ site does not appear to be due to DNA binding, as the clamp loader forms an avid complex with β at a 5′ site. Preferential loading at a 3′ versus a 5′ site occurs at the ATP hydrolysis step, needed to close the ring around DNA. We also address DNA structural features that are recognized for preferential loading at a 3′ site. Although the single-stranded template strand extends in opposite directions from 3′ and 5′ primed sites, thus making it a favorite candidate for distinguishing between 3′ and 5′ sites, the single-strand polarity at a primed template junction does not determine 3′ site selection for clamp loading. Instead, we find that clamp loader recognition of a 3′ site lies in the duplex portion of the primed site, not the single-strand portion. We present evidence that the β clamp facilitates its own loading specificity for a 3′ primed site. Implications to eukaryotic clamp loader complexes are proposed.     

The Titer of the Lentiviral Vector Encoding Chimeric TRIM5α-HRH Gene is Reduced Due to Expression of TRIM5α-HRH in Producer Cells and the Negative Effect of Ef1α Promoter

Molecular Biology - The chimeric protein TRIM5α-HRH is a promising antiviral factor for HIV-1 gene therapy. This protein is able to protect cells from HIV-1 by blocking the virus in the...     

恒河猴TRIM5α的组织分布及不同刺激促进PBMC中TRIM5α转录水平的上调

TRIM5α(tripartite motif protein 5-alpha)蛋白是恒河猴体内一种非常重要的限制因子,能抑制人免疫缺陷病毒(HIV-1, human immunodeficiency virus type 1)、马感染性贫血病毒(EIAV, equine infectious anemia virus)和猫免疫缺陷病毒 (FIV, feline immunodeficiencyvirus)等逆转录病毒的复制。恒河猴TRIM5α的组织分布以及在受到外界刺激时TRIM5α mRNA表达量的变化研究还未见报道。本研究从中国恒河猴的各组织中提取总RNA,以β-actin基因作为内参照,通过半定量RT-PCR检测各组织中TRIM5α mRNA的表达。选择HIV-GFP-VSVG假病毒感染外周血单核细胞（peripheral blood mononuclear cell,PBMC）,非特异性刺激剂——佛波脂(Phorbol myfismte acetate,PMA)+离子霉素(ionomycin,Ion)及CD28抗体+CD49d抗体分别共刺激恒河猴PBMC,研究不同刺激对恒河猴TRIM5α mRNA表达水平的影响。结果表明：TRIM5α mRNA表达于所研究的恒河猴21种组织中,免疫系统和泌尿生殖系统组织中表达量最高,而神经系统组织,如大脑、脊髓中表达较少,其他组织中未见明显的表达差异;HIV-GFP-VSVG感染和用PMA+Ion、CD28抗体+CD49d抗体分别共刺激PBMC能促进 PBMC中TRIM5α mRNA的转录水平的上调。     

熊猴存在TRIM5/TRIMCyp杂合子基因型

缺乏合适的动物模犁是制约艾滋病研究取得重大突破的关键瓶颈之一.细胞内的抗病毒蛋白被称为限制因子.研究不同灵长类动物抗HIV-1宿主限制因子的存在形式及作用机制对建立合适AIDS灵长类动物模型有十分重要的意义.TRIM5α是哺乳动物细胞中一种重要和关键的限制因子,它以物种依赖的方式限制包括HIV-1在内的逆转录病毒的感染.TRIM5-CypA融合基因是存在于新大陆猴与旧大陆猴中的一种独特的TRIM5基因形式.为了研究不同灵长类动物TRIM5基因的存在方式,该文对熊猴、藏婀猴、红面猴及中闰恒河猴4个物种共110只灵长类动物进行了TRIM5-CypA融合模式的研究.首次发现熊猴也存在TRIM5-CypA基因融合现象.熊猴TRIMCyp融合基因形成模式类似于北平顺猴TRIMCyp融合基因模式,即CypA假基因的cDNA序列通过逆转座方式插入到TRIM5基凶的3'-UTR区域.基因序列分析表明,该基因与北平顶猴相应基因序列高度相似;并且其TRIM5内含子6的3'-剪接位点也相应存存G-to-T突变现象(G/T).这提示熊猴也极有可能像北平顶猴一样表达TRIM5-CypA融合蛋白,从而导致熊猴可能跟北平顶猴一样可能被HIV-1感染.因此,熊猴极有希望成为一种新的HIV/AIDS灵长类动物模型.